Welding method and apparatus

ABSTRACT

In a method for welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules ( 100 ), each tube is loaded into a holder ( 52 ), brought to a welding station ( 80 ), has one end welded closed and is then released from the holder ( 52 ). A number of holders can be arranged on the periphery of a rotatable member ( 50 ), each holder being loaded with a tube at a first station, and then carrying the tube to a welding station ( 80 ) and an ejection station ( 90 ). The tubes can be loaded into the holders individually by means of an escapement mechanism ( 40 ). The invention also extends to apparatus for carrying, out the method

[0001] The present invention relates to a welding method and apparatus,and more particularly to a method and apparatus for automated welding oftubes for use in brachytherapy.

[0002] In the treatment of various cancers, and in particular prostatecancer, a process called brachytherapy has proved effective. Inbrachytherapy, small capsules containing radioactive material areimplanted in or near to the tumour.

[0003] One known form of capsule or canister, commonly used to treatprostate cancer and referred to as a “seed”, is shown in FIG. 7. Thecapsule 100 comprises a silver rod 102, coated with a radioactiveisotope of iodine such as I-125, inside a hollow titanium tube 104. Theends of the tube are welded closed. Resin balls coated with radioactiveiodine can be used instead of the silver rod 102. The completed capsulehas a width of approximately 1.0 mm and a length of approximately 4.5mm. The capsules or seeds can be implanted into a patient individually;alternatively, the capsules can be inserted into medical stitchingmaterial or suture, which is then inserted into the prostate and leftthere.

[0004] The number of capsules implanted into each patient obviouslyvaries in accordance with the regime of treatment required, but iscommonly in the region of 50 to 100. The capsules are normally made byhand, with the welding process used to close the ends of the tube beingcarried out manually. It will be appreciated that making such a largenumber of capsules for each patient by hand takes considerable time andexpense.

[0005] According to a first aspect of the invention, there is providedan automated method of welding closed one end of each of a multiplicityof open-ended tubes suitable for use in the manufacture of brachytherapycapsules, which comprises repeating the steps of loading each tube intoa holder, bringing the tube in the holder to a welding station, weldingclosed one end of the tube, and releasing the welded tube from theholder.

[0006] With such a method, a large number of tubes can be processedrelatively quickly.

[0007] Preferably, the step of loading each tube into a holder comprisesthe steps of orientating the tubes into a particular orientation in afeeder, and feeding each tube from the feeder to a holder. Onceorientated in the feeder, the tubes can be fed to the holder in adesired orientation. Manual picking up and orientation of the tubes isavoided.

[0008] As the tubes are for use in making brachytherapy seeds, they willbe of a suitable size for brachytherapy implantation, for example, a fewmillimetres long by about one millimetre wide. The tubes are typically alittle longer than the product brachytherapy seed, since it is usual forsome of the tube material to be used to form the weld closure.

[0009] The welding of one end of the tube closed is preferably effectedby melting the material of the tube such that the melted material formsthe end closure. Preferably, no additional material is required.

[0010] Closure of the remaining open end of the tube will be effectedafter the active substance for brachytherapy has been inserted.

[0011] In general, the tubes to be welded will be formed by being cutfrom a long length of tubing, and having been so cut they will berandomly orientated. According to the method of the invention, the tubesare orientated in the feeder, so that each tube is ready to be fed to aholder. A preferred method comprises agitating the tubes in order toorientate them in the feeder.

[0012] Various ways may be used to control feeding of each tube to aholder. For example each tube may be individually picked up by a robotarm and placed in the holder. Preferably, the method comprises arrangingthe tubes end to end one above another in the feeder, urging arestraining member against a side of the lowermost-but-one tube toprevent it from downward movement, and releasing the lowermost tube fromthe feeder individually to the holder. The restraining member thusengages the lowermost-but-one tube and in so doing all the tubes aboveare held in position, allowing the lowermost tube to be individuallyreleased. After release, a blocking member is preferably moved into ablocking position below the tubes and the restraining member isretracted to allow the tubes to drop down, the then lowermost tube beingstopped by the blocking member. The restraining member can then beadvanced against the now lowermost-but-one tube and the feeding cycle isthen repeated.

[0013] In a preferred method, the holder receives a tube, conveys thetube to the welding station, and releases the tube, said holder movingin a closed loop.

[0014] The provision of a holder which moves in a closed loopfacilitates the automation of the welding process. The tube to be weldedis supplied to the holder in a particular orientation. The tube is thencarried to the welding station where it is welded. The welded tube isreleased, preferably at an ejection station to which it is carried fromthe welding station. Since the holder moves in a closed loop, it comesback to its original position, and the procedure can then be repeated.

[0015] It is possible to use one holder, although the rate of productionof such an apparatus is low, as it can only operate on a single tube ata time. Accordingly, it is preferred that a plurality of said holdersare provided. This allows greater productivity.

[0016] The welding of the end of each tube obviously requires hightemperatures in order to melt the metal of the tube. It is possible forthese high temperatures to cause damage to whatever is holding the tubeduring the welding operation, particularly if repeated weldingoperations are carried out. Some form of cooling can be provided toreduce the risk of damage. However, in a preferred method, the holdersmove through said welding station sequentially, each of the plurality ofholders cooling between successive welding operations while the otherholders move through the welding station. This ensures that after eachof the holders holds a tube while it is being welded, it cools downbefore it next holds a tube being welded. This reduces the risk of heatdamage to the holders.

[0017] Many possible methods of moving the or each holder through thewelding station can be employed. For example, each could be individuallydriven along a track.

[0018] However, in a preferred method, the or each holder is mounted atthe periphery of a rotatable member. Receiving, welding and releasingstations may conveniently be provided at successive points along therotational path of the holder. Where a plurality of holders are providedthey can all be mounted by the rotatable member, with the advantage ofsimplicity, as it is then only necessary to provide a single drivemeans.

[0019] The holder may for example be a cavity into which the tube can bedropped by the feeder and which restricts movement of the tubeadequately during welding. Preferably, however, the holder adopts anopen condition when receiving and releasing a tube and a closedcondition to grip the tube firmly at least during welding of the tube.This arrangement assists the receiving and releasing steps, whilstproperly holding the tube during welding.

[0020] The holder may be in the closed condition at all times exceptwhen receiving and releasing a tube. However, the method preferablycomprises causing the holder to move from the open condition to theclosed condition when the holder is at the welding station.

[0021] Thus, means can be provided at the welding station for causingthe closing action, rather than such means being provided as part of theholder itself. This can advantageously simplify the design of theholder, which is particularly advantageous where the holder is conveyedfrom one processing station to another, e.g. by a rotatable member.

[0022] Viewed from another aspect the invention provides an automatedmethod of welding closed one end of each of a multiplicity of open-endedtubes suitable for use as brachytherapy capsules, comprising repeatingthe steps of feeding each individual tube to a holder in an opencondition, conveying the holder in the open condition to a weldingstation, causing the holder to close and grip the tube firmly by meansof a holder actuator provided at the welding station, and welding oneend of the tube closed. The holder actuator is advantageously notconveyed with the holder but instead is provided at a fixed location atthe welding station.

[0023] The holder may have a floor portion for preventing downwardmovement of the tube during conveyance. However, this is not necessaryand the method preferably comprises sliding the tube on a slide plateduring conveyance of the holder. It has been found, contrary toexpectations, that the lower ends of the tubes are not damaged as aresult of sliding along the slide plate.

[0024] Thus, as long as the tubes are of uniform lengths, the top end ofthe tube will always be the same height above the slide plate. Providedthe welder is also fixed in position, the distance between the upper endof the tube and the welding unit will remain constant. This can ensurethat the tube is in the correct position for welding. Thus it isstrongly preferred that the multiplicity of tubes for use in thepreferred embodiment of the present invention are of uniform length.

[0025] It is possible for the welded tubes (also referred to as “cans”)to be removed from the holders by being picked out, either by hand or byproviding a further piece of apparatus to carry out this step. However,it is preferred that the slide plate be provided with a cut-out region,allowing cans to fall past the slide plate. The cans are then removedfrom the holders by the force of gravity. A preferred method thereforecomprises conveying the welded tube to a release position where theslide plate is provided with a cutout region, allowing the can to fallthrough the cut-out region.

[0026] The holder can take a number of forms, as long as the tube isheld correctly. For example, the tube could be held in an inflatablecollar, which is normally deflated and can be inflated when it isnecessary to hold the tube tightly. However, it is preferred that saidholder is in the form of a pair of jaws which are biassed to an openposition and can be selectively closed. This allows the tube to beinserted into the holder when they are in their open position, where thespacing between the jaws is relatively wide, and so facilitatesinsertion of the tube. This also allows for release of the tube byopening the jaws.

[0027] It is possible for the jaws themselves to be provided with meansallowing them to be closed tightly around the tube. For example, asolenoid or a pneumatic cylinder could have its ends attached to thehalves of the jaw, and would then be able to open or close the jaws e.g.at the welding station. However, as described above, a holder actuatoris preferably provided at the welding station to close the jaws. Thissimplifies the construction of the jaws, and, where a plurality of setsof jaws are provided, involves the provision of just one holderactuator, rather than one for each set of jaws.

[0028] In a preferred method, the holder is in the form of a blockslidably mounted in a recess in a holder support, said block beingbiassed outwardly by resilient means, e.g. compression springs, retainedbetween said block and said holder support. This arrangement has theadvantage that the holder can be closed tightly by pushing the blockagainst the resilient bias, and this can be done by means such as asolenoid or an air cylinder.

[0029] Preferably, the block is retained on the holder support by boltspassing through the block and engaging in the holder support, the blockbeing biassed outwardly by the resilient means, and the bolts retainingthe block on the holder support against the resilient bias and thusserving to define the open condition of the block, which can be variedby tightening or loosening the bolts. When the block is in its openposition, it is biassed away from the recess, and so the distancebetween the block and the recess is a maximum. This is the positionwhere the tube will preferably be loosely held in the holder. The boltscan be tightened or loosened to change this spacing, for example toaccommodate thicker or thinner tubes, which may be needed to makedifferent types of capsule.

[0030] The spacing between the block and the recess is important, as itmust be large enough to allow the tube to easily enter the holder, butsufficiently small to ensure that the tube is held as required. Inparticular, the tube must be held in such a way as to ensure that whenthe holder is closed tightly around it, it is in the correct positionfor welding. The spacing can be set by inserting a gauge member of knownthickness into the gap and tightening the bolts until the jaws justcontact the gauge member, but this is an awkward procedure and does notguarantee that the bolts will not be over tightened. Accordingly, it ispreferred that a set screw is provided in said recess, said set screwdefining a point beyond which said bolts may not be tightened. Thisensures that the spacing of the block and the recess is set correctly.

[0031] Provision of the set screw to ensure correct spacing of the blockand the recess is also considered to be of inventive merit, and soaccording to a further aspect of the invention there is provided anautomated method of welding closed one end of each of a multiplicity ofopen-ended tubes suitable for use as brachytherapy capsules, comprisingrepeating the steps of receiving a tube in a holder mounted on a holdersupport, conveying the tube past a welding station where one end of thetube is welded closed in a welding operation, and releasing the weldedtube, said holder being in the form of a block slidably mounted in arecess on the holder support and retained on said holder support bybolts passing through said block and engaging in said holder support,said block being biassed outwardly by resilient means acting betweensaid block and said holder support, said bolts retaining said block onthe holder support against the resilient bias and thus serving to definean open condition of the block, said open condition of said block beingvariable by means of tightening or loosening said bolts, and wherein aset screw is provided in said recess to define a point beyond which saidbolts may not be tightened.

[0032] Since the holder will hold the tube while it is being welded, itis necessary for the holder to be formed from a material which hassufficient heat-resistance. It is preferred that the or each holder isformed from Elkonite. This material, an alloy of tungsten and copper,has good heat-resistance, but is easier to work than tungsten itself.

[0033] It is also preferred that the holder support, e.g. rotatablemember, is formed from Elkonite.

[0034] The invention also extends to apparatus for carrying out themethods for welding closed one end of each of a multiplicity of tubesfor use as brachytherapy capsules as described above.

[0035] A preferred embodiment of the invention will now be provided byway of example only and with reference to the accompanying drawings, inwhich:

[0036]FIG. 1 is an overall perspective view of the preferred embodimentof the automated tube welder;

[0037]FIG. 2 is an enlarged perspective view of the feed line and theescapement assembly;

[0038]FIG. 3 is a perspective view of the escapement assembly;

[0039]FIG. 4 is a perspective view of the turret;

[0040]FIG. 5 is an enlarged exploded perspective view of the turret;

[0041]FIG. 6 is a perspective view of the table plate and slide plate;and

[0042]FIG. 7 is a cross-sectional view of a brachytherapy capsule orseed.

[0043]FIG. 1 shows a general perspective view of the preferredembodiment of the automated tube welder 10. The titanium tubes which areused to form the capsules have a width of around 1.0 mm and a length ofaround 6.5 mm. The tubes arrive in a bowl 22 of a vibrating hopper. Thevibration of the hopper serves to orientate the tubes, and they passinto a gravity feed line 30. The tubes pass through to the lower end ofthe gravity feed line, where they are stopped by an escapement assembly40. The escapement assembly 40 allows only one tube through at a time,and retains the remaining tubes in a vertical stack in the feed line.The tube released by the escapement assembly falls into a jaw 52 of agenerally circular turret assembly 50. The turret 50 has fourcircumferentially equispaced jaws 52, each of which can hold one tube,and the turret 50 is rotated to move the tube sequentially through anumber of operating stations. The first station is the one where thetube is received by the jaws. The turret is then rotated by 90° twice,to bring the tube to a welding station 80 incorporating a welding unit84. A sensor 82 is located at or near the welding station, and as theturret rotates the sensor detects the presence or absence of a tubesticking up from the turret. The welding unit is only operated if thepresence of the tube has been detected by the sensor assembly; if a tubeis detected, then the upper end of the tube is welded closed by thewelding unit. Once the welding process is started, it follows a presetwelding program, which ends with a signal being sent back indicatingthat the welding process is complete. If the welding program is startedand the weld sequence is not completed, the process will terminate witha weld error signal. If, on the other hand, no tube is detected, thenthe welding unit is not activated. The turret 50 is then rotated by afurther 90° to bring the welded tube or can to an ejection station 90,where it is ejected. The operation of the automated tube welder 10 iscontrolled by a control unit, which in the preferred embodiment is anAllen Bradley PLC 5/11 programmable logic controller, using RS Logixsoftware from Allen Bradley.

[0044] The various parts of the tube welder will now be described ingreater detail.

[0045] The bowl 22 of the vibrating hopper 20 receives the tubes, anddelivers them lengthwise down an exit chute 24. The width of the chute24 is such that only one tube can pass down the chute at a time. This isachieved by having the diameter of the chute slightly greater than thediameter of the tubes (which is approximately 1 mm) but substantiallyless than the length of the tubes (which is around 6.5 mm), so that anin-line arrangement of the tubes results. Hoppers of this type areavailable from Service Engineering Inc., and the hopper will not bedescribed further.

[0046] The upper end of the gravity feed line 30 is attached to thelower end of the exit chute 24, and the tubes leave the exit chute 24and enter the feed line 30 under gravity. The width of the teed line 30is similar to that of the chute 24, so that only one tube can pass downthe feed line at any one time. The tubes thus form a vertical column inthe feed line, stacked end to end.

[0047] In addition, the inner surfaces of the chute and the feed lineare formed to be as smooth as possible. This reduces the frictionbetween the tubes and the chute and feed line, and so reduces the chanceof the tubes jamming during feeding.

[0048] At the lower end of the feed line 30 is the escapement assembly40, which ensures that the tubes leave the feed line one by one. Theescapement assembly includes a plate 42, which normally blocks thepassage of tubes from the feed line 30, such that the lowermost tuberests on the plate 42. The escapement assembly also includes a resilientmember 44, which normally urges the tube just above the lowermost tubeagainst the side of the feed line 30, with sufficient force to preventthis tube from moving downwards when the tube below it moves, but not somuch force that the tube will be damaged.

[0049] It will be appreciated that the walls of the tubes are very thin,so as to minimize any attenuation of the radiation from the radioisotopesuch as radioactive iodine, and it is preferred that the walls have athickness of around 0.05 mm. Excessive force can damage the tubes, bycrushing the walls towards each other, and this is obviouslyundesirable. For this reason, the resilient member is preferably in theform of a spring which is easily compressed.

[0050] The plate 42 and the resilient member 44 are moved by aircylinders, controlled by the control unit.

[0051] In operation, the escapement assembly 40 will normally have theplate 42 positioned so as to block the passage of tubes from the end ofthe feed line 30. The stack of tubes will thus be supported on theplate. The resilient member 44 then presses the lowermost-but-one tubeagainst the side of the feed line 30, to prevent it from moving. Theplate 42 is then moved to allow the lowermost tube to fall. It ispossible for the plate to be formed with an opening which is broughtinto and out of registration with the opening at the bottom of the feedline, but it is preferred for the entire plate to be moved out of theway, to ensure that the lowermost tube is reliably dropped.

[0052] When the plate 42 is moved to allow the lowermost tube to dropfrom the feed line, it will be appreciated that the weight of all of theother tubes in the stack is supported by the tube which is pressedagainst the wall of the feed line 30 by the resilient member 44.However, since each tube has a very low mass (of the order of 10 mg) thetotal mass which must be supported in this way is relatively small.

[0053] The plate 42 is then returned to its original position, and theresilient member 44 is withdrawn. This allows the entire stack of tubesto drop down, so that they are again supported on the plate 42. Eachtime this cycle is repeated, one tube is allowed to drop from the bottomof the feed line 30. The operation of the escapement assembly 40 iscontrolled by the control unit. In addition, an interlock in theescapement 40 prevents both the plate 42 and the resilient member 44allowing the tubes to pass at the same time.

[0054] Directly below the plate 42 of the escapement assembly 40, thereis a funnel member 32. The funnel has a passage therethrough to allow atube to pass down it. The purpose of the funnel is to ensure that thetube which is allowed out of the feed line 30 by the escapement assembly40 is directed accurately, rather than simply allowed to fall. This isimportant, as if the tube were to simply be dropped from a height abovethe jaws 52 of the turret assembly 50, it could tumble out of itsvertical orientation and as a result may not be properly received by thejaws 52.

[0055] In addition, a sensor unit 46 detects the presence of each tubeas it passes through the escapement assembly 40. If the sensor fails todetect any tubes passing through the escapement assembly for a certainperiod of time during the operation of the apparatus; then an errormessage is generated by the control unit, and the user is informed ofthe location of the problem. The most likely causes for the errormessage being generated are that the tubes have jammed somewhere in thefeed line 30 or that there are no tubes remaining in the hopper 20. Ineither case, it is necessary for the operator of the apparatus tointervene.

[0056] As mentioned above, each tube that drops from the feed line 30falls into a set of jaws 52 mounted on the turret assembly 50. There arefour such sets of jaws 52, equispaced around the circumference of theturret 50. The turret itself is formed from a flat generally circularplate 54, rotatably mounted and driven by an indexing motor in such away that it rotates in steps of 90°. The plate is made of atungsten/copper alloy called Elkonite, which has a higher melting pointthan titanium. This is vital because a defined region of each titaniumtube has to be at least partially melted during the welding process toform the weld, which involves relatively high temperatures. It ispossible to form the turret from tungsten, rather than an alloy thereof,but tungsten is brittle and difficult to work, and thus forming theturret from tungsten presents a number of difficulties. In addition,Elkonite has the desirable property of heating up slowly but coolingquickly.

[0057] The turret 50 is rotatably positioned above a table plate 70,shown in FIG. 6. It will be seen from this Figure that the centralregion of the table plate 70 over which the turret 50 rotates isremoved, to allow the shaft of the indexing motor to pass through thetable plate 70 and be attached to the turret 50 in order to drive it. Inaddition, the part of the table plate over which the turret 50 rotatesis formed as a recess which accommodates a slide plate 72. The slideplate is formed of a material which has good wear resistance and goodsliding qualities, and in the preferred embodiment is formed ofaluminium bronze. The slide plate and the portion of the table plate 70which it overlies are continuous around most of their circumferentialextent, but have a cut-out region 74 in one portion. The purpose of thiscut-out region will be described later.

[0058] Each set of jaws 52 comprises a block 56 which is accommodated ina recess 58 in the plate 54. As best shown in FIGS. 4 and 5, two sets ofjaws 52 are positioned at the ends of a first diameter of the plate 54,and the other two sets are mounted at the ends of the diameter normal tothe first diameter. Each block 56 is mounted on guide pins 60 so as tobe slidable in the recess 58 towards and away from the centre of theplate along the diameter. The block 56 is held in place by two bolts 62which extend through the block parallel to the diameter of the plate,and engage in holes formed in the plate. Springs 64 surround the bolts62 and are positioned between the recess 58 and the block 56 to bias theblock outwardly until the block comes into contact with the heads of thebolts.

[0059] The end of the block 56 which faces the recess 58, and the end ofthe recess 58 which faces the block 56, are spaced from each other, andthe space between the end of the block and the end of the recess definesthe region into which the tube falls. In order to adjust the size of theregion into which the tube falls, the bolts 62 can be tightened againstthe springs 64 (to reduce the size of the region) or loosened, so thatthe springs 64 push the block outwardly (to increase the size of theregion).

[0060] During the setting-up of the turret 50, the bolts 62 aretightened to set the width of the jaws 52. This can be done by putting agauge member of known thickness into the jaws and tightening the jawsuntil they just contact the gauge. However, this is an awkward andinaccurate procedure. To ensure that the jaw spacing is set correctly, aset screw 66 is provided in the recess 58 to ensure that the bolts 62cannot be tightened beyond a set position. However, the springs 64 andguide pins 60 are arranged so that the block 56 can still slide into therecess 58 beyond this point, to reduce the width of the jaws.

[0061] The ends of the block 56 and the recess 58 are both formed with asemicylindrical groove 68. The radius of the groove 68 is very slightlyless than the radius of the tube. When the bolts 62 have been tightenedas far as they can, the distance between the ends of the block 56 andthe recess 58 is set to be around 0.5 mm, which is less than thethickness of a tube. However, a tube can be easily accommodated betweenthe semicylindrical grooves 68, and indeed there is substantial playbetween the grooves 68 and the walls of the tube.

[0062] Rotation of the turret 50 brings the tube held in the jaws 52firstly to a welding station 80, located 180° away from the escapement40, and then to an ejection station 90 90° further on.

[0063] It will be noted that there is no station positioned at 90° fromthe escapement assembly. However, if the process requires, a stationcould be put here to carry out an optional step useful to prepare thetube for welding. For example, an air blast could be used to blow anydebris away from the end of the tube.

[0064] The welding station 80 comprises a sensor 82, a welding unit 84,and a clamping air cylinder 86. The welding unit 84 comprises a plasmaarc welding torch, in which argon is used as an inert gas. Supply ofcurrent and argon gas to the welding torch is regulated by the welderpower supply. The position of the welding head 88 of the welding unitcan be adjusted in all three dimensions to ensure that it is properlyaligned with the upper end of the tube.

[0065] The sensor assembly 82 detects the presence or absence of a tubein the jaws 52 of the turret 50. An optical detection system is used,and a signal from the sensor 52 is passed to the control unit. If thesensor detects a tube, then the welding unit 84 welds the upper end ofthe tube closed. If no tube is detected, then the welder is notoperated. Of course, the sensor 82 could be positioned at the previous(empty) station if desired.

[0066] The clamping air cylinder 86 serves to close the jaws 52 of theturret 50. The piston of the clamping air cylinder 86 can be operated tomove horizontally towards the centre of the turret when a set of jaws 52are positioned at the welding station 80. When the piston moves in thismanner, it applies a force to the block 56, pushing it into the recess58. This clamps the tube between the groove 68 in the block 56 and thegroove 68 in the recess 58, and thus ensures that the tube is accuratelypositioned for welding. It also ensures that there is good contactbetween the tube and the jaws, which is necessary for successfulwelding. The operation of the clamping air cylinder 86 is controlled bythe control unit.

[0067] It will be appreciated that since the faces of the block andrecess come together after the block has travelled a very short distanceinto the recess, the block has limited travel. As a result, it isextremely unlikely that the tube will be crushed or otherwise damaged bythe jaws while it is being held for welding.

[0068] A quarter of a revolution further on is the ejection station 90.This station is positioned adjacent the parts of the table plate 70 andslide plate 72 which have the cut-out region 74. As the turret 50rotates, the welded tube or can is brought to a position above thecut-out region 74. Normally, since the can is only held loosely in thejaws 52, it will fall from the jaws 52 under the influence of gravity,and will pass through the cut-out region 74 and down a delivery chute 92into a collection bin (not shown)

[0069] On occasion, the can does not fall from the jaws 52 under gravitywhen it is positioned above the cut-out region 74. For this reason, anejection unit is provided, comprising a vertical air cylinder 94, withthe piston of the air cylinder biassed into its uppermost position bymeans of a spring. An ejector block 96 is attached to the lower end ofthe piston. When air is supplied to the cylinder 94, the piston ispushed downwardly, and the lower surface of the ejector block 96contacts the upper end of the can. This suffices to push the candownwardly out of the jaws 52 into the delivery chute 92, and ensuresthat the cans are reliably ejected. Supply of air to the cylinder 94 isregulated by the control unit.

[0070] Further, although not shown in the diagrams, the welder isentirely enclosed by a plastic safety cage. The safety cage reduces therisk of operator injury (whether through welder burns or throughtrapping a finger or similar between moving parts). Further, the safetycage allows excess argon to build up near the welder to improve welding.In addition, the safety cage protects the welding area from external aircurrents, and reduces the amount of airborne contaminants near thewelder.

[0071] The safety cage is provided with doors, to allow access to thewelder when necessary (for example, when charging the hopper 20 withtubes). The doors have switches, which pass a signal to the control unitindicating whether the doors are closed or not. For automatic operationof the welder, the doors must be closed.

[0072] The operation of the automated tube welder will now be described.Tubes are placed into the bowl 22 of the vibratory hopper 20, and aredelivered down the exit chute 24 into the gravity feed line 30. Thetubes form a vertical stack, with the lowermost tube resting on theplate 42 of the escapement assembly 40. The turret 50 is positioned suchthat a set of jaws 52 is aligned beneath the bottom of the funnel 32 ofthe gravity feed line 30, to receive a tube therefrom, and is heldstationary in that position. The block 56 of the jaws 52 is biassedoutwardly by the springs 64, and so the space between the grooves 68 inthe block 56 and the recess 58 is somewhat larger than the cross-sectionof the tube. This facilitates entry of the tube into the jaws 52.

[0073] The resilient member 44 of the escapement assembly 40 then urgesthe lowermost-but-one tube in the stack against the side of the feedline 30. The plate 42 of the escapement assembly 40 is then moved toallow the lowermost tube in the stack to fall through the funnel 32 intothe jaws 52. The tube released in this way falls under gravity into thegap between the grooves 68 in the block 56 and the recess 58, until itslower end comes into contact with the slide plate 72 beneath the turret50. The length of the tube is slightly greater than the thickness of theturret, and so the upper end of the tube stands proud of the turret.

[0074] The plate 42 of the escapement assembly 40 is then returned toits original position, and the resilient member 44 releases the tubewhich it was holding, which then drops down to rest on the plate 42.

[0075] The turret is then rotated by 90°. The tube is carried along withthe jaws 52, and its lower end slides on the slide plate 72. The weightof the tube is sufficiently small for the lower end of the tube to avoidbeing damaged as it slides on the slide plate 72. As there is no stationpositioned 90° from the escapement, no operations are carried out on thetube at this stage in the process. In the meantime, while the turret 50is stationary, the escapement assembly 40 is operated again, to drop atube into the second set of jaws.

[0076] The turret 50 is again rotated by 90°. The first tube is broughtto the welding station 80, where its presence is detected by the sensor82. The second tube is moved to the empty station.

[0077] A signal that a tube is present is passed to the control unit,which then prepares the welding unit 84 to make a weld. At the weldingstation 80, the air cylinder 86 is activated by the control unit toclamp the first tube firmly in position in the jaws 52, and while it isheld in this way its upper end is welded closed. Following welding, thepressure in the air cylinder 86 is released, and the jaws 52 spring backto their previous position. Of course, if no tube is present in the jaws52 (as will occur in the stages before the first tube reaches thewelding station 80) then the sensor 82 passes a signal to this effect tothe control unit, and the welder is not activated.

[0078] It will be appreciated that during the welding of the upper endof the tube, it is important for the turret 50 to be held stationary, toensure that the tube remains beneath the welder. During this stationaryperiod, the escapement assembly 40 is operated again to drop a tube intothe third set of jaws 52.

[0079] The turret is again rotated by 90°. The first tube, by now weldedclosed at one end, is brought to the ejection station 90, where it fallsinto the delivery chute 92, either under its own weight or after beingtapped by the ejector block 96. The second tube is brought to thewelding station 80, where its presence is detected, the jaws 52 areclosed by the air cylinder 86 and the end of the second tube is weldedclosed. The third tube is carried along by the jaws 52 but nothing isdone to it. The escapement assembly 40 is again operated while theturret 50 is stationary during the welding process to drop a tube intothe fourth set of jaws 52.

[0080] The turret is again rotated by 90°. This brings the second tubeto the ejection station 90, the third tube to the welding station 80,and the fourth tube to the empty station. The escapement assembly 40 isoperated again to drop a tube into the original set of jaws 52, whichare now empty.

[0081] The process then continues, with each tube being dropped into aset of jaws 52 and conveyed through the welding station 80, where oneend of the tube is welded 30 closed, to the ejection station 90, whereit is ejected. The apparatus thus continuously and automatically weldsclosed one end of each of the titanium tubes.

[0082] The time taken to complete a weld is the critical time in thecycle, which determines how quickly the welder can operate. At present,completion of a weld takes approximately three seconds, and rotation ofthe turret by 90° takes roughly two seconds. Each 90° step delivers awelded tube to the ejection station 90, and the production rate isaround 600 welded tubes or cans per hour.

[0083] It will be noted that, although the welding unit 84 is operatedin each cycle, each set of jaws 52 only holds the tube being weldedevery fourth cycle. This allows the material forming the turret 50 to beless heat-resistant than would be necessary if it were always the samepart holding the tube being welded. In particular, it allows the turretto be formed from a tungsten alloy rather than tungsten itself.

[0084] The welded titanium tubes or cans produced by this process arereceived in a collection bin. They are subjected, to a visualinspection, and those that are acceptable are filled with theappropriate radioisotope such as silver rods or resin balls coated withradioactive iodine. The capsules then have their other end weldedclosed. At present, the inspection, insertion and welding steps arecarried out manually. However, it will be appreciated that even if thesesteps are carried out manually, having the first end of the tube weldedclosed automatically significantly reduces the amount of work and timeinvolved in the production of the capsules. In addition, the automatedprocess produces more consistent results than the manual welding ofcans.

1. An automated method of welding closed one end of each of amultiplicity of open-ended tubes suitable for use as brachytherapycapsules, which comprises repeating the steps of: loading each tube intoa holder; bringing the tube in the holder to a welding station; weldingclosed one end of the tube; and releasing the welded tube from theholder.
 2. A method as claimed in claim 1, wherein the multiplicity ofopen-ended tubes are fed in a continuous stream from a feeder.
 3. Amethod as claimed in claim 2, wherein the feeder supplies the tubes in aparticular orientation to a holder.
 4. A method as claimed in claim 3,comprising agitating the tubes in order to orientate them in the feeder.5. A method as claimed in any of claims 2 to 4, comprising arranging thetubes end to end one above another in the feeder, urging a restrainingmember against a side of the lowermost-but-one tube to prevent it fromdownward movement, and releasing the lowermost tube from the feederindividually to the holder.
 6. A method as claimed in any precedingclaim, wherein the holder receives a tube, conveys the tube to thewelding station where one end of the tube is welded, and releases thewelded tube, said holder moving in a closed loop.
 7. A method as claimedin claim 6, wherein a plurality of said holders are provided.
 8. Amethod as claimed in claim 7, wherein said holders move through saidwelding station sequentially, each of said plurality of holders coolingbetween successive welding operations while the other holders movethrough said welding station.
 9. A method as claimed in any precedingclaim, wherein said holder is mounted at the periphery of a holdersupport in the form of a rotatable member.
 10. A method as claimed inany preceding claim, wherein said holder adopts an open condition whenreceiving and releasing a tube and a closed condition to grip the tubefirmly at least during welding of the tube.
 11. A method as claimed inclaim 10, comprising causing the holder to move from the open conditionto the closed condition when the holder is at the welding station.
 12. Amethod as claimed in claim 10 or 11, wherein said holder is in the formof a pair of jaws which are biassed to an open position and can beselectively closed.
 13. A method as claimed in any of claims 10 to 12,wherein said holder is in the form of a block slidably mounted in arecess in a holder support, said block being biassed outwardly byresilient means retained between said block and said holder support. 14.A method as claimed in claim 13, wherein said block is retained on saidholder support by bolts passing through said block and engaging in saidholder support, the block being biassed outwardly by the resilientmeans, and the bolts retaining the block on the holder support againstthe resilient bias and thus serving to define the open condition of theholder, which can be varied by tightening or loosening said bolts.
 15. Amethod as claimed in claim 14, wherein a set screw is provided in saidrecess, said set screw defining a point beyond which said bolts may notbe tightened.
 16. An automated method of welding closed one end of eachof a multiplicity of open-ended tubes suitable for use as brachytherapycapsules, comprising repeating the steps of feeding each individual tube-to a holder in an open condition, conveying the holder in the opencondition to a welding station, causing the holder to close and grip thetube firmly by means of a holder actuator provided at the weldingstation, and welding one end of the tube closed.
 17. A method as claimedin claim 16, comprising sliding the tube on a slide plate duringconveyance of the holder.
 18. A method as claimed in claim 17,comprising conveying the welded tube to a release position where theslide plate is provided with a cut-out region, and allowing the tubes tofall through the cut-out region.
 19. An automated method of weldingclosed one end of each of a multiplicity of open-ended tubes suitablefor use as brachytherapy capsules, comprising repeating the steps ofreceiving a tube in a holder mounted on a holder support, conveying thetube past a welding station where one end of the tube is welded closedin a welding operation, and releasing the welded tube, said holder beingin the form of a block slidably mounted in a recess on the holdersupport and retained on said holder support by bolts passing throughsaid block and engaging in said holder support, said block being biassedoutwardly by resilient means acting between said block and said holdersupport, said bolts retaining said block on the holder support againstthe resilient bias and thus serving to define an open condition of theblock, said open condition of said block being variable by means oftightening or loosening said bolts, and wherein a set screw is providedin said recess to define a point beyond which said bolts may not betightened.
 20. A method as claimed in any preceding claim, wherein theor each holder is formed from Elkonite.
 21. A method as claimed in anypreceding claim, wherein a holder support is formed from Elkonite. 22.Apparatus for carrying out a method as claimed in any of the precedingclaims.
 23. A method for welding closed the ends of tubes for use asbrachytherapy capsules substantially as described herein with referenceto FIGS. 1 to
 6. 24. Apparatus for welding closed the ends of tubes foruse as brachytherapy capsules substantially as described herein withreference to FIGS. 1 to 6.